The present invention generally relates to the preparation of semiconductor material substrates, especially silicon wafers, which are used in the manufacture of electronic components. More particularly, the present invention relates to a single crystal silicon wafer comprising an epitaxial silicon layer with reduced autodoping and a back surface that is free of halo.
In the production of single silicon crystals grown by the Czochralski method, polycrystalline silicon is first melted within a quartz crucible with or without dopant. After the polycrystalline silicon has melted and the temperature equilibrated, a seed crystal is dipped into the melt and subsequently extracted to form a single crystal silicon ingot while the quartz crucible is rotated. The single crystal silicon ingot is subsequently sliced into individual silicon wafers which are subjected to several processing steps including lapping/grinding, etching, and polishing to produce a finished silicon wafer having a front surface with specular gloss. In addition to polishing the front surface, many device manufacturers also request a polished back surface with a specular gloss (such wafers are commonly referred to as xe2x80x9cdouble-side polishedxe2x80x9d). To prepare the finished wafer for device manufacturing, the wafer may be subjected to a chemical vapor deposition process such as an epitaxial deposition process to grow a thin layer of silicon generally between about 0.1 xcexcm and about 200 xcexcm thick on the front surface of the wafer such that devices can be fabricated directly on the epitaxial layer. Conventional epitaxial deposition processes are disclosed in U.S. Pat. Nos. 5,904,769 and 5,769,942.
The epitaxial deposition process is typically comprised of two steps. In the first step after the silicon wafer is loaded into a deposition chamber and lowered onto a susceptor, the front surface of the wafer is subjected to a cleaning gas such as hydrogen or a hydrogen/hydrochloric acid mixture at about 1150xc2x0 C. to xe2x80x9cpre-bakexe2x80x9d and clean the front surface of the silicon wafer and remove any native oxide on that surface to allow the epitaxial silicon layer to grow continuously and evenly onto the front surface. In the second step of the epitaxial deposition process the front surface of the wafer is subjected to a vaporous silicon source such as silane or trichlorosilane at about 800xc2x0 C. or higher to deposit and grow an epitaxial layer of silicon on the front surface. During both steps of the epitaxial deposition process the silicon wafer is supported in the epitaxial deposition chamber by the susceptor which is generally rotated during the process to ensure even growth of the epitaxial layer. The susceptor is generally comprised of high purity graphite and has a silicon carbide layer completely covering the graphite to reduce the amount of contaminants such as iron released from the graphite into the surrounding ambient during high temperature processes. Conventional susceptors used in epitaxial growth processes are well known in the art and described in U.S. Pat. Nos. 4,322,592, 4,496,609, 5,200,157, and 5,242,501.
During the loading process, gas can be trapped between a conventional susceptor and the wafer as the wafer is lowered onto the susceptor causing the wafer to xe2x80x9cfloatxe2x80x9d and slide onto the susceptor in a position that is not intended (e.g., partly out of the recessed xe2x80x9cpocketxe2x80x9d). This can result in uneven epitaxial growth. Furthermore, during the pre-bake step a small amount of cleaning gas such as hydrogen can effuse around the wafer edge between the wafer and the susceptor and into the space between the wafer and the susceptor. If the back surface of the wafer is sealed with an oxide layer (typically about 3000 xc3x85 to about 5500 xc3x85 thick), the effused hydrogen will not react sufficiently with the oxide layer to create pinholes in the layer or completely remove the oxide layer. If the back surface is an etched or polished surface as desired by many device manufacturers and only has a thin native oxide layer (typically about 15 xc3x85 to about 30 xc3x85), the hydrogen or hydrogen/hydrochloric acid mixture will typically completely remove the native oxide layer near the outer edge of the back surface where the cleaning gas effuses around the wafer and create pinhole openings in the native oxide layer exposing the silicon surface as etching moves inward from the outer edge of the wafer. These pinhole openings typically form in an annular region inward of the circumferential edge of the wafer.
During the epitaxial deposition process a small amount of silicon containing source gas can also effuse around the wafer edge between the wafer and the susceptor and into space between the wafer and the susceptor. If the back surface of the wafer is oxide sealed, nucleation and growth of a silicon film is substantially suppressed. In areas where the native oxide layer has been completely etched away by the cleaning gas a smooth continuous layer of silicon is grown. However, in areas where the cleaning gas has not completely removed the native oxide layer, pinholes in the native oxide layer expose the silicon wafer and allow the silicon containing source gas to deposit silicon in the pinholes and create a nonuniform silicon film on the wafer backside during the epitaxial deposition. Thus, for wafers with etched or polished back surfaces having only a native oxide layer, pinholes created in the native oxide layer during the pre-bake step may lead to discontinuous silicon growth on the back surface which appears hazy under bright light illumination. This haziness or xe2x80x9chaloxe2x80x9d on the back surface of the wafer is comprised of small silicon growths or bumps having a diameter of about 0.5 xcexcm and being about 10 nm high. These bumps of silicon scatter light and lead to haziness and can be deemed undesirable as they can interfere with machine vision and optical pyrometry systems that view the back surface of the wafer during device processing. The halo is particularly visible to the eye under bright light and by laser surface scanners on the specular glossy back surface of a double side polished wafer (see FIG. 12A). In contrast, the relatively rough back surface of a single side polished wafer results in a significant degree of diffuse scattering of reflected light which reduces the appearance of halo.
Another problem encountered during the high temperature growth of the epitaxial silicon layer is the out-diffusion of dopant atoms such as boron or phosphorus through the back surface of the silicon wafer during the high temperature pre-bake and the epitaxial growth steps. With conventional susceptors, the dopant atoms that out-diffuse from the back surface can effuse between the wafer edge and the susceptor toward the front surface of the wafer. These dopant atoms can be incorporated into and contaminate the growing deposition layer and degrade the resistivity uniformity near the wafer edge. If the back surface of the silicon wafer is oxide sealed, the dopant atoms will not substantially out-diffuse from the back surface. Silicon wafers having etched or polished back surfaces, however, are subject to out-diffusion of dopant atoms from the back surface during the epitaxial deposition process which can lead to unwanted autodoping of the front surface.
Several methods have been suggested for attempting to eliminate back surface halos and autodoping. To eliminate back surface halos Nakamura (Japanese Unexamined Patent Application No. JP11-16844) disclosed performing a hydrogen fluoride strip and/or a high-temperature hydrogen annealing step of the back surface up to 10 days before the wafers are loaded into the epitaxial reactor. The process adds additional processing steps which can greatly increase complexity and cost of the deposition process. Deaton et al. (U.S. Pat. No. 5,960,555) disclosed a method of preventing the frontside reactive source gas from effusing to the wafer backside by utilizing a susceptor with built-in channels along the wafer edge for directing purge gas flows to the edge of the wafer. This process requires substantial modification of existing epitaxial deposition chambers and utilizes increased purge gas flows which can cause the purge gas to spill over to the front surface and mix with the source gas which can degrade the resulting epitaxial film.
To reduce autodoping, Hoshi (Japanese Unexamined Patent Application No. JP11-87250) disclosed using vacuum sucking on the edge of a susceptor to evacuate boron dopant on the edge of the susceptor and prevent autodoping. This process may affect wafer edge uniformity and thickness and requires substantial modification to existing epitaxial deposition systems. Nakamura (Japanese Unexamined Patent Application JP10-223545) disclosed a modified susceptor having slots on the edge of the susceptor such that the out-diffused dopant atoms would be pushed down through the slots and into the exhaust. This method also allows a substantial amount of the deposition gas to be evacuated below the back surface of the wafer which can lead to the halo affect previously discussed as well as premature corrosion of the exhaust system and safety concerns.
To date, therefore, methods of controlling the halo effect on the back surface of silicon wafers and autodoping problems associated with dopant out-diffusion from the back surface during an epitaxial deposition process have not been satisfactory. As such, a need exists in the semiconductor industry for a simple, cost effective approach to solving the halo effect and unwanted autodoping of the front surface of a silicon wafer during an epitaxial deposition process.
Among the objects of the present invention, therefore, is the provision of a single crystal silicon wafer which (a) has an epitaxial surface that is essentially unaffected by gas-phase autodoping; and (b) has a back surface free from halo.
Briefly, therefore, the present invention is directed to a single crystal silicon wafer comprising a silicon wafer substrate having a central axis, a front surface and a back surface which are generally perpendicular to the central axis, a circumferential edge, and a radius extending from the central axis to the circumferential edge of the wafer. The back surface of the wafer is free of an oxide seal and substantially free of a chemical vapor deposition process induced halo. Additionally, the silicon wafer substrate comprises P-type or N-type dopant atoms. The single crystal silicon wafer further comprises an epitaxial silicon layer on the front surface of the silicon wafer substrate. The epitaxial silicon layer is characterized by an axially symmetric region extending radially outwardly from the central axis toward the circumferential edge wherein the resistivity is substantially uniform. The radius of the axially symmetric region is at least about 80% of the length of the radius of the substrate. The epitaxial silicon layer also comprises P-type or N-type dopant atoms.
This invention is also directed to a process for growing an epitaxial silicon layer on a silicon wafer substrate in a chemical vapor deposition chamber. The process comprises contacting the front surface of the silicon wafer substrate and substantially the entire back surface of the silicon wafer substrate with a cleaning gas to remove an oxide layer from the front surface and the back surface of the silicon wafer substrate. After the oxide layer is removed, the epitaxial layer is grown on the front surface of the silicon wafer substrate. During the growth of the epitaxial layer, a purge gas is introduced into the the chemical vapor deposition chamber to reduce the number of out-diffused dopant atoms from the back surface of the silicon wafer substrate incorporated in the epitaxial silicon layer.
This invention is also directed to an apparatus for the support of a silicon wafer during the growth of an epitaxial silicon layer via a chemical vapor deposition process. The apparatus comprises a susceptor sized and configured for supporting the silicon wafer thereon. The susceptor has a surface with a density of openings between about 0.2 openings/cm2 and about 4 openings/cm2 which is in a generally parallel opposed relationship with the silicon wafer. The openings permit fluid flow through the surface for fluid contact with the back surface of the silicon wafer.
This invention is also directed to an apparatus for use in an epitaxial deposition process wherein an epitaxial silicon layer is grown on a silicon wafer substrate with a front surface and a back surface. The apparatus comprises a chamber, a wafer support device for supporting the silicon wafer substrate and rotatable means for supporting the wafer support device and the silicon wafer substrate. The wafer support device permits fluid contact with the front surface of the silicon wafer substrate and substantially the entire back surface of the silicon wafer substrate. The apparatus further comprises a heating element, a gas inlet for allowing cleaning gas, source gas and purge gas to enter the apparatus and a gas outlet for allowing the foregoing gases to exit the apparatus.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.